1. Field of the Invention
This invention relates generally to the preparation of submicron-sized noble metal alloy particles. More particularly, the present invention relates to a method for forming fine powdered alloys of platinum and rhodium with a discrete size and shape. The method comprises: 1) forming a dilute solution of soluble metal salts to form soluble solute aggregates consisting of a joint metal salt species, 2) fast freezing droplets of the dilute solution in a liquid cryogen in order to separate the solution droplets into solute-rich and solvent-rich phases, 3) subliming of the frozen droplets to remove the frozen solvent from both phases to provide a joint metal salt powder, and, 4) reducing the powder to provide a joint metal species.
Applications for which the present invention may be useful include the formation of catalysts such as those used in electrochemical applications, including fuel cells and the like, automotive applications such as automotive catalytic converters, and other similar applications. Also useful are application wherein the metal particles are used to form inks which may be laid down and subsequently processed to form electrically conductive pathways such as on ceramic integrated circuit (IC) chip carriers. Finally, these materials may be used as the feedstock for forming near net shaped parts using electrochemical or laser assisted deposition and/or consolidation techniques.
The instant application, therefore, is drawn to the chemical arts, in general, and the catalysis arts and nano-particle arts, in particular. Accordingly, it is appropriate to search these fields for art which might prove to be of significance in determining the relevance and uniqueness of the approach taken by the instant invention.
Regarding the catalyst arts, the instant invention is particularly drawn to methods for forming small particles, either as a pure powder which is later dispersed onto a support means or as particles suspended directly onto the support means. Commonly known methods for loading catalysts particles onto catalyst support means include precipitation, adsorption, ion exchange, and impregnation. Precipitation is typically carried out such that a support means, comprising powders or particles, is mixed into a metal salt solution at a concentration sufficient to obtain the required loading of the catalyst onto the support; for example, as a metal hydroxide or carbonate, and to cause interaction of the precipitate with the support surface. Numerous steps may be required to control the size of the precipitated particles and the amount of precipitate loaded onto the support. Preparation of alloyed particles is particularly problematic.
Adsorption is carried out by exposing support to salt solutions wherein the support typically adsorbs some quantity of the salt ions. Adsorption of salt ions from solution may be either cationic or anionic depending on the properties of the support surface.
The ion exchange process for loading a metal catalyst onto a support is similar to the adsorption process. For example, ion exchange support means have been developed whereby the ion exchange material is prepared with a specific concentration of one species, such as sodium, which is washed with a solution of a second species, such as ammonium, resulting in NH+4 ions being exchanged for Na+ions. The treated ion exchange support means can be soaked in a solution containing the metal catalyst, and the metal catalyst may then be loaded onto the ion exchange support means. The extent of metal catalyst loading depends on, among other things, the soaking time and the concentration of sodium on the ion exchange material.
In impregnation processing, metal salt solutions are typically incorporated into pores which are present in support means. The support means can be heated or evacuated to remove moisture in the pores and to accelerate the rate of diffusion of the salt into the pores. Drying is typically required to crystallize the salt on the surface, and may result in irregular concentrations or distributions of the metal salt. Non-uniformity of particle concentration and distribution typically results.
In preparing catalysts by the techniques discussed above, after the metal salt is deposited onto a support means, the metal salt typically is reduced to an active phase, such as an active phase comprising an oxide or a pure metal. Thermal decomposition procedures are frequently used and typically occur in a reducing atmosphere, such as hydrogen gas. Thermal decomposition procedures may be beneficial only when used in combination with support means which can withstand the temperatures associated with the thermal decomposition process (for example, the support means should not be adversely modified by such thermal decomposition processes). Examples of high temperature materials suitable as support means includes carbon and various ceramic materials. Thus, the effectiveness of the purely thermal decomposition technique is limited to supports or substrates which can withstand the temperatures associated with thermal decomposition. Thermal decomposition processes may be further limiting where the metal catalysts themselves can be adversely affected by high temperatures. Furthermore, the exceedingly high temperatures required for alloying xe2x80x9crefractoryxe2x80x9d metals typically lead to further difficulties regarding contamination of the alloy by the containing vessel, etc.
Alternative techniques for decomposing a metal salt into an active phase include direct chemical reducing techniques. Reducing techniques may comprise reducing agents such as, hydrazine or sodium tetrahydroborate and the like, which may be used to reduce the metal salt to a metal catalyst.
In the formation of nano-particles, the instant invention is drawn to a unique method for forming alloys. In particular, synthesis of the particles from xe2x80x9cfirst principles,xe2x80x9d i.e., assembling atoms into clusters, is most easily accomplished with single phase systems and becomes much more difficult as the system becomes more complex. Specifically, creating alloy particles of refractory metals without the prerequisite high temperature remains a non-trivial hurdle. One of the difficulties comprises maintaining the proper composition throughout the synthesis to avoid producing particles with varying composition. Another is the preparation of materials which are not contaminated by either their surroundings or the process used to achieve synthesized material, i.e., attrition (comminution) of melt produced alloys.
2. Description of Related Art
U.S. Pat. No. 5,686,150 discloses a process for depositing metals onto various substrates. In particular the patent teaches various salts of platinum, palladium and ruthenium deposited on substrates such as organic polymers, hybrid polymers and carbons. Said salts are then reduced by exposing the substrate with an energy source and then contacting the exposed substrate to a developer to permit reduction of the metal.
U.S. Pat. No. 5,514,202 discloses a method for producing fine alloy powders of silver and palladium at temperatures at or below 50xc2x0 C. The method comprises forming solutions of the metal nitrates, mixing them, and then adding a neutralizing and complexing agent to the mixed solution to adjust the solution pH to between 2.5 and 3.5. This resultant solution comprises a silver and palladium ions. A solution comprising a reducing agent is then prepared and brought into contact with the mixed solution while stirring the solutions and maintaining the solution temperature between 15xc2x0 C. and 50xc2x0 C. in order to allow the silver and palladium ions to be reduced and to co-precipitate and form silver-palladium alloy particles.
U.S. Pat. No. 4,721,524 teaches a method for producing a non-pyrophoric submicron alloy powder of Group VIII metals. An aqueous chemical process involving spontaneous nucleation is employed in the manufacture of the alloy product. One such product is prepared with isomorphous compounds of nickel and palladium. In the preferred form, small concentrations of palladium and/or platinum ions are added to ionic nickel and/or cobalt solutions. A hot solution of the metal ions of nickel and/or cobalt and palladium and/or platinum is mixed with hydrazine in a hot basic aqueous solution and rapidly diluted with hot water. The precipitate formed is filtered, sequentially chemically washed, and dried. The dried alloy product is a black, non-pyrophoric, magnetic powder. Examination by electron probe and X-ray diffraction shows the alloy product to be a solid solution of the constituent metals.
U.S Pat. No. 4,145,214 discloses a photo-conductor composition that can be prepared by a co-crystallization technique. In particular, this invention teaches that the photo-conductor can be formed by co-crystallization of the two components p-terphenyl and p-quaterphenyl, from solution in a common solvent. Typically the co-crystalline material is precipitated, for example, by subsequent evaporation of the solvent.
U.S. Pat. No. 3,357,819 discloses a process for preparing homogenous powders composed of ultra-fine particles. A solution or dispersion of a salt is freeze dried by dripping into a cold solution such as liquid nitrogen, followed by sublimation of the water from the particles. The patent does not refer to any pre-process method for controlling the shapes of the particles so generated.
Many other examples exist of alloy formation by similar solution-precipitation processes but, as will be shown, none exhibit the essential characteristics of the instant invention.
It has been discovered that certain organic compounds of platinum, and palladium can be dissolved into common organic solvents, caused to precipitate, and be reduced to the constituent metal at temperature below 200xc2x0 C. to yield very small and essentially uncontaminated metal particles.
It also has been discovered that the presence of these materials in co-location with particles of other Group VIIIA noble metal will provide the same benefit; that the presence of platinum or palladium particles in close proximity to these other noble metal materials will aid in their low temperature reduction.
It also has been discovered that certain organic compounds of platinum and palladium, can be caused to co-crystallize with compounds of rhodium, iridium, rhenium, and ruthenium having the same organic ligand, and then reduced to a fine metal alloy at temperatures below 200xc2x0 C.
The instant invention comprises a process which provides a pure noble metal alloy at temperatures below 150xc2x0 C. The process yields high purity, small particles, typically in the submicron size range.
It is, therefore, an object of this invention to provide a process for preparing finely divided metal alloy at temperature near room temperature.
Another object of this invention is to provide a process for preparing finely divided, unagglomerated, alloy particles free of the contamination usually associated with solution or attrition processing powders.
Another object of this invention is to provide a process for preparing a noble metal alloy by dissolving an a metal-containing organic form of the desired product into a compatible solvent to induce formation of species complexes which can than be rapidly cooled in a cryogenic liquid nitrogen bath and thereby co-crystallizing the multiple solute complex species.
Yet another object of this invention is to provide a process for producing a metal powder which greatly reduces or eliminates post-process handling and thus greatly reducing the possibility of post-process contamination.
It is a further object of this invention to provide a low temperature process for making an electrode containing noble metal alloy particles uniformly dispersed on or throughout the electrode substrate.
Yet another object of this invention is to provide a low temperature method for producing a catalyst in situ, the catalyst comprising a porous support and a noble metal alloy uniformly dispersed throughout the support.
Another object of this invention is to provide free-flowing metal and metal alloy powders having a submicron particle size.
A final object of this invention is to provide a metal ink comprising submicron sized noble metal particles.
To achieve these and other objects, there is provided a method for producing noble metal and noble metal alloy powders at low temperature. It should be noted, however, that the instant invention is not limited solely to the a metal-containing organic compound disclosed herein but is applicable to any combination comprising platinum or platinumxe2x80x94like metals.